Mercedes-Benz 300D Whining sound when car is in gear Inspection at your home or office.

Common reasons for this to happen:

• Low Transmission Fluid: For both manual and automatic transmissions, the primary cause for whining when in gear is low transmission fluid. If the fluid is too low, then the internal components of the transmission are not lubricated properly. What you’re hearing is actually friction between those parts, and it can lead to significant damage. If the fluid is low, it’s advised to check for transmission fluid leaks.

• Normal Operation: As mentioned, some transmissions have an inherent whine that is completely normal. Depending on the make and model, the whine might be primarily audible in first gear, or it could be more noticeable in third gear, or some other gear. The best defense here is to know what your transmission sounds like normally.

• Worn Clutch and/or Flywheel: If you drive a manual transmission, there’s the possibility that your clutch is worn out and/or your flywheel needs to be resurfaced. When these components begin to wear, it’s possible to hear whining, grinding and other abnormal sounds.

• Bad Throw Out Bearing: If you’re hearing the whine while the clutch is not engaged, there’s a chance that it’s the throw out bearing. However, it’s more common to hear noise from this bearing while operating the clutch, rather than driving in gear without any pressure on the clutch pedal.

• Bad Input Shaft Bearing: This is an internal bearing within the housing of the transmission, and it can create a whining sound when it begins to fail.

• Bad Wheel Bearing: Wheel bearings are located in each wheel hub, but the sound can travel and seem as though it’s coming from the transmission. It’s more usual for a failing wheel bearing to create a roaring sound, but it’s possible for them to create a whine while driving.